What is a good term to denote someone who is obsessed with, or erudite in, geography?
Geographer on Wikipedia: A geographer is a scientist whose area of study is geography, the study of Earth's physical environment and human habitat.
I'd suggest that, while this doesn't exactly apply to obsessives, it certainly would apply to the erudite and those who are generally very interested in geography.
Geographile (deprecated) or Geographophile!
If one is going to create a word, one might as well go for the more direct route.
While I couldn't find a respected source for this nonce word (Geographile), a Google search did result in >12k results (likely, in part, to several social media platforms having a user with this moniker.)
Geographophile, on the other hand, produced exactly "About 7 results" from Google, but may be the more correct, if rarer, nonce word.
Use either at your own pleasure / risk. :-)